The present invention relates to a combustion method for NOx reduction, as well as an apparatus therefor, to be applied to water-tube boilers, reheaters of absorption refrigerators, or the like.
Generally, as the principle of suppression of NOx generation, there have been known (1) suppressing the temperature of flame (combustion gas), (2) reduction of residence time of high-temperature combustion gas, and (3) lowering the oxygen partial pressure. Then, various NOx reduction techniques to which these principles are applied are available. Examples that have been proposed and developed into practical use include the two-stage combustion method, the thick and thin fuel combustion method, the exhaust gas recirculate combustion method, the water addition combustion method, the steam jet combustion method, the flame cooling combustion method with water-tube groups (water-tube cooling combustion method), and the like.
With respect to small-size once-through boilers, as of today, there has been laid out in Tokyo Metropolis or others a regulation that the exhaust NOx value of gas-fired boilers should be not more than 60 ppm (at 0% O2 in the exhaust gas, dry basis; hereinbelow, the unit ppm is expressed at 0% O2 in the exhaust gas, dry basis, unless otherwise specified), and that the exhaust NOx value of oil-fired boilers should be not more than 80 ppm for A-type heavy oil and not more than 60 ppm for kerosine. Many manufacturers including the present applicant have cleared these regulation values. However, California in U.S.A. has already laid out a regulation specifying not more than 12 ppm (at 3% O2 in the exhaust gas, dry basis). The applicant, envisaging that even stricter regulations, e.g. not more than 30 ppm, will be applied in the near future also in Japan, has been performing research and development for further NOx reduction.
A prior-art NOx reduction techniques is proposed in combinations of above-described various suppression principles (see, e.g., Patent Reference 1: Japanese Published Patent Application H07-103411, Page 3, FIG. 1). This prior-art technique is a combination of the exhaust gas recirculate technique and the steam jet. However, with this NOx reduction technique, it is not easy to achieve an exhaust NOx value of not more than 30 ppm (hereinafter, referred to as xe2x80x9ctarget exhaust NOx valuexe2x80x9d).
That is, the present inventors of this application have found through various experiments and discussions that the following issues exist in order to achieve the target exhaust NOx value or lower in the prior art.
First, in the prior art, for reduction of the NOx value by a functional enhancement of combustion gas temperature suppression with the exhaust gas recirculation, the functional enhancement is to increase the exhaust-gas recirculation quantity. However, implementing this functional enhancement would cause unstable characteristics of the exhaust gas recirculation to be amplified. That is, the exhaust gas recirculation has a characteristic that the exhaust-gas flow rate or temperature changes with changes in combustion quantity or changes in load. An increase in the exhaust-gas recirculation quantity would cause these unstable characteristics to be amplified, making it impossible to achieve a stable NOx reduction. Also, an increase in the exhaust-gas recirculation quantity would cause the oxygen concentration in the combustion air to lower, resulting in a combustion state of oxygen deficiency, so that the combustion could no longer be continued because of incomplete combustion or discharge of unburned combustibles. Further, a volume increase corresponding to the exhaust-gas recirculation quantity would cause the pressure loss in the air blow passage to increase, thus making it inevitable to increase the cost due to the increase in the blower capacity.
Also, a functional enhancement of NOx reduction by steam addition is to increase the quantity of water to be added. This functional enhancement would cause an increase in thermal loss and moreover an increase in the quantity of condensations, posing a problem of corrosion of the constituent equipment due to the condensations.
An object of the present invention is to provide a combustion method for NOx reduction, as well as an apparatus therefor, capable of solving these and other issues and easily achieving NOx reduction with the value of exhaust NOx under 30 ppm.
The present invention having been accomplished to solve the above object, in a first aspect of the invention, there is provided a combustion method for NOx reduction comprising in combination the steps of: a first NOx reduction step for suppressing generated NOx value to 60 ppm or under (at 0% O2 in exhaust gas, dry basis) by a low NOx burner; a second NOx reduction step for recirculating exhaust gas of the low NOx burner to a burning reaction zone formed by the low NOx burner; and a third NOx reduction step for adding water or steam to the burning reaction zone.
In a second aspect of the invention, there is provided a combustion method for NOx reduction as described in the first aspect, wherein the second NOx reduction step is performed with a target exhaust NOx value set to 30 ppm or under (at 0% O2 in exhaust gas, dry basis) and with an exhaust-gas recirculation quantity set in a stable combustion range of the low NOx burner, and any NOx value exceeding the target exhaust NOx value is reduced by the third NOx reduction step.
In a third aspect of the invention, there is provided a combustion method for NOx reduction as described in the first or second aspect, wherein the third NOx reduction step is performed by spraying water directly to the burning reaction zone.
In a fourth aspect of the invention, there is provided a combustion apparatus for NOx reduction, comprising: a low NOx burner for suppressing generated NOx value to 60 ppm or under (at 0% O2 in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NOx burner to a burning reaction zone formed by the low NOx burner; and water or steam addition means for adding water or steam to the burning reaction zone.
Further, in a fifth aspect of the invention, there is provided a combustion apparatus for NOx reduction, comprising: a low NOx burner for suppressing generated NOx value to 60 ppm or under (at 0% O2 in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NOx burner to a burning reaction zone formed by the low NOx burner; and water spraying means for spraying water directly to the burning reaction zone.
In one embodiment, there is provided a NOx reduction combustion method as described in any one of the first to third aspects, wherein the NOx reduction step is performed with an excess air ratio which is determined from a NOx reduction target value and an excess air ratio versus NOx characteristic of the NOx reduction step.
Before the description of embodiments of the present invention, terms used herein are explained. The combustion gas includes burning-reaction ongoing (under-combustion-process) combustion gas, and combustion gas that has completed burning reaction. Then, the burning-reaction ongoing gas refers to combustion gas that is under burning reaction, and the burning-completed gas refers to combustion gas that has completely burning-reacted. The burning-reaction ongoing gas is indeed a concept of substance, but can also be referred to as flame as a concept of state because it generally includes a visible flame so as to be in a flame state. Therefore, herein, the burning-reaction ongoing gas is referred to also as flame or burning flame from time to time. Further, the burning reaction zone refers to a zone where the burning-reaction ongoing gas is present, and the exhaust gas refers to burning-completed gas that has decreased in temperature under an effect of endothermic action by heat transfer tubes or the like.
Also, the combustion gas temperature, unless otherwise specified, means the temperature of burning-reaction ongoing gas, equivalent to combustion temperature or combustion flame temperature. Further, the suppression of combustion gas temperature refers to suppressing the maximum value of combustion gas (combustion flame) temperature to a low one. In addition, normally, burning reaction is continuing although in a trace amount even in the burning-completed gas, and so the combustion completion does not mean a 100% completion of burning reaction. The target exhaust NOx value refers to a target value for the NOx value exhausted from the NOx reduction combustion apparatus.
Next, embodiments of the present invention are described. The present invention is applied to thermal equipment (or combustion equipment) such as small-size once-through boilers or other water-tube boilers, water heaters, reheaters of absorption refrigerators or the like. The thermal equipment has a burner and a group of heat absorbers to be heated by combustion gas derived from the burner.
An embodiment of the method according to the present invention is a combustion method for NOx reduction comprising in combination the steps of: a first NOx reduction step for suppressing generated NOx value to 60 ppm or under, preferably 50 ppm or under, by a low NOx burner; a second NOx reduction step for recirculating exhaust gas of the low NOx burner to a burning reaction zone formed by the low NOx burner; and a third NOx reduction step for adding water or steam to the burning reaction zone. Means for performing the first NOx reduction step, means for performing the second NOx reduction step, and means for performing the third NOx reduction step are referred to as first NOx reduction means, second NOx reduction means, and third NOx reduction means, respectively.
The first NOx reduction means is the low NOx burner. The low NOx burner may be implemented by a burner that suppresses the generated NOx value to 60 ppm or under by using any one or combining any ones from among the divided flame combustion method, the self recirculate method, the staged combustion method, the thick and thin fuel combustion method, and other techniques. The low NOx burner is preferably given by a gas-fired burner, but may also be an oil-fired burner in another embodiment.
Then, burning reaction is performed in front of the low NOx burner, by which a burning reaction zone is formed.
The second NOx reduction means is what is called exhaust gas recirculation method, in which part of exhaust gas to be discharged into the atmospheric air after having decreased in temperature under an effect of endothermic action by the heat absorbers is mixed into the combustion air by external recirculation via an exhaust-gas recirculation passage, which is an external passage. By a combustion-gas-temperature suppression effect or a decrease in oxygen concentration or the like attributable to this mixed exhaust gas, the NOx value is reduced.
The exhaust-gas recirculation quantity by the second NOx reduction means is set to within the stable combustion range of the low NOx burner. The stable combustion range refers to a range in which the exhaust CO amount is 100 ppm or under, preferably 50 ppm or under.
The third NOx reduction means is water or steam addition to the burning reaction zone. By this water or steam addition, the burning-reaction ongoing gas is cooled so that the combustion gas temperature is suppressed, thus the NOx value being reduced.
The water or steam addition is performed, preferably, by spraying water directly toward the burning reaction zone. By doing so, in an embodiment in which gaseous mixture of combustion air and exhaust gas is blown to the low NOx burner by a blower, it becomes implementable to prevent the blower from corrosion and to fulfill the NOx reduction while suppressing the increase in the capacity of the blower to a minimum.
Otherwise, the water or steam addition by the third NOx reduction means may be done in the exhaust-gas recirculation passage in another embodiment. Furthermore, in an embodiment in which the gaseous mixture of combustion air and exhaust gas is fed to the low NOx burner by a blower, steam addition may be performed between the low NOx burner and the blower.
In the combustion method for NOx reduction of this embodiment, the target exhaust NOx value is set to 30 ppm or under, preferably 20 ppm or under. Then, the generated NOx value by the first NOx reduction means is set to 60 ppm or under, preferably 50 ppm or under, and subsequently a NOx reduction is performed by the second NOx reduction means.
With this arrangement, given that the generated NOx value by the first NOx reduction means is A, the NOx reduction value by the second NOx reduction means is B and the target exhaust NOx value is X, then it is assumed that the third NOx reduction means fulfills a NOx value of Axe2x88x92Bxe2x88x92X=C. That is, setting the NOx reduction value by the third NOx reduction means to C or more makes it possible to achieve the target exhaust NOx value or under.
By this method as described above, there can be produced an effect that the target exhaust NOx value or under can be achieved without incurring the aforementioned issues of the exhaust gas recirculation, and moreover such problems as the corrosion of the equipment can be avoided and further the increase in the blower capacity can be suppressed to a minimum.
Also, in the foregoing embodiment, preferably, a combustion space where the heat transfer tubes are not present, i.e. the heat transfer tubes have been eliminated, is formed in front of the low NOx burner, so that the burning reaction is performed in the combustion space, with a burning reaction zone formed there. Desirably, the combustion space has such an area that burning reaction of the fuel jetted out from the low NOx burner is completed within the zone, but this is not limitative.
That a combustion space where the heat transfer tubes are not present is formed in front of the low NOx burner means that the water-tube cooling combustion method is not aggressively performed. As a result of this, it is no longer necessary to take measures for the issues of the water-tube cooling combustion method, i.e., the emission of large amounts of CO or unburned combustibles due to the burning-reaction suppression effect of the water tubes. In particular, the NOx reduction technique by the water-tube cooling combustion method has an issue that the combustion itself cannot be continued in applications to combustion apparatus using an oil-fired burner, and therefore it is preferable to form in front of the low NOx burner a combustion space where the heat transfer tubes are not present.
Further, in the foregoing embodiment, preferably, the water or steam addition is performed by spraying water directly toward the burning reaction zone within the combustion space. By doing so, a stable suppression of the combustion gas temperature is fulfilled. Also, in the embodiment in which the gaseous mixture of combustion air and exhaust gas is blown to the low NOx burner by a blower, it becomes implementable to prevent the blower from corrosion and moreover to prevent the blower from increasing in load.
Next, embodiments of the apparatus according to the present invention are described. The present invention includes the following embodiments (1) to (5) of the apparatus corresponding to the foregoing embodiments of the method.
Embodiment (1): A combustion apparatus for NOx reduction comprising: a low NOx burner for suppressing generated NOx value to 60 ppm or under (at 0% O2 in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NOx burner to a burning reaction zone formed by the low NOx burner; and water or steam addition means for adding water or steam to the burning reaction zone.
Embodiment (2): A combustion apparatus for NOx reduction as defined in Embodiment (1), wherein with a target exhaust NOx value of 30 ppm, the target exhaust NOx value is fulfilled by NOx reduction effects by the exhaust gas recirculation means and the water or steam addition means.
Embodiment (3): A combustion apparatus for NOx reduction comprising: a low NOx burner for suppressing generated NOx value to 60 ppm or under (at 0% O2 in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NOx burner to a burning reaction zone formed by the low NOx burner; and water spraying means for spraying water directly to the burning reaction zone.
Embodiment (4): A combustion apparatus for NOx reduction, wherein a combustion space where the heat transfer tubes have been eliminated is formed in front of the low NOx burner.
Embodiment (5): A combustion apparatus for NOx reduction comprising: a low NOx burner for suppressing generated NOx value to 60 ppm or under (at 0% O2 in exhaust gas, dry basis), the low NOx burner being switchable between low combustion and high combustion; exhaust gas recirculation means for recirculating exhaust gas of the low NOx burner to a burning reaction zone formed by the low NOx burner in low combustion and high combustion of the low NOx burner; and water or steam addition means for adding water or steam to the burning reaction zone only in the high combustion of the low NOx burner.